This invention relates to a single stage hydrorefining process for treating heavy oils using two particular catalysts arranged in a particular manner, referenced to herein as "stacked bed". It particularly relates to a single stage hydrorefining process for treating oils having a tendency to deactivate hydrotreating catalysts by coke formation, these being oils with high boiling components and/or oils with a low asphaltene content and very high boiling components, with a particular stacked bed catalyst arrangement. The use of the stacked bed increases the catalyst life or allows increased conversions relative to the more traditional catalysts used for the treating of these oils. The invention is particularly useful for meeting the demands of increasing hydrotreatment severity, such as sulfur removal, for poorer quality heavy oil fractions both directly distilled or extracted from crude or crude fraction and oil fractions from thermal, steam, or catalytic cracking processes including mixtures of any of these materials.
The continual changes in the refining industry such as the trend to poorer quality crudes and the continual increase in the stringency of oil product specifications (e.g. lower allowed sulfur content) is in part requiring the refiner to increase the severity of hydrotreating of traditional oil fractions and/or process fractions not traditionally treated. The increased severity and/or unusual feed generally have been causing increased deactivation of hydrotreating catalysts. The invention herein disclosed can be used with these oil fractions to increase the run length of a hydrotreating process and/or allow higher severity operation and/or process poorer quality oils.
The use of lower price or locally available crudes frequently results in increased sulfur and/or nitrogen content of the oil fractions. Conversion processes such as thermal cracking, coking and catalytic cracking are either being brought on-stream or are processing poorer quality oils. The products from such processes are laden with heteroatoms such as sulfur and are more hydrogen deficient relative to products from better quality crudes or oils distilled directly from crude or crude fractions. As a result, the products of the conversion processes and/or poorer crudes have to be hydrotreated to meet specifications or to prepare for further treating/conversion. However, the higher operating temperatures required to remove the additional heteroatoms and add additional hydrogen in addition to the hydrogen-deficient coke-like nature of these feeds results in increasing deactivation of the hydrotreating catalysts due to coking. Any increase in hydrotreating catalyst activity and/or stability enables refiners to upgrade the lower value poor quality and/or cracked oils at a significant economic benefit.
It is well known that the hydrogen-deficient poor quality oils can be hydrotreated/hydrorefined with low catalyst deactivation rates at higher hydrorefining unit conditions--higher hydrogen pressure, and/or hydrogen-to-oil ratio, and/or oil-catalyst contact time. To stay within the physical or design constraints of the unit or continue to process the required volumes of oil, only relatively small variations in these parameters can be made. As a result, very expensive hydrotreating equipment must be added to meet the changing goals unless catalysts with longer lives are available. Alternatively, the refiner can accept very short catalyst lives and increased down time for frequent catalyst changes or use continuous or semicontinuous regeneration facilities. Larger and/or more vessels and additional equipment would be needed to process a given quantity of feed stock with these options. Of particular importance to a refiner is the ability to process the hydrogen-deficient and/or poorer quality oils in existing hydrotreating units which do not have sufficient hydrogen pressure to prevent uneconomically rapid catalyst activity loss with existing catalysts utilized in a non-stacked bed configuration. Thus, improved processes and highly stable catalysts are in great demand.
Several two-stage hydrotreating processes have been proposed to overcome some of the difficulties of hydrotreating heavy oils. The five patents discussed below use two catalyst reactor vessels, and are incorporated herein by reference.
U.S. Pat. No. 3,766,058 discloses a two-stage process for hydrodesulfurizing high-sulfur vacuum residues. In the first stage some of the sulfur is removed and some hydrogenation of feed occurs, preferably over a cobalt-molybdenum catalyst supported on a composite of ZnO and Al.sub.2 O.sub.3. In the second stage the effluent is treated under conditions to provide hydrocracking and desulfurization of asphaltenes and large resin molecules contained in the feed, preferably over molybdenum supported on alumina or silica, wherein the second catalyst has a greater average pore diameter than the first catalyst.
U.S. Pat. No. 4,016,049 discloses a two-stage process for hydrodesulfurizing metal- and sulfur-containing asphaltenic heavy oils with an interstage flashing step and with partial feed oil bypass around the first stage.
U.S. Pat. No. 4,048,060 discloses a two-stage hydrodesulfurization and hydrodemetallization process utilizing a different catalyst in each stage, wherein the second stage catalyst has a larger pore size than the first catalyst and a specific pore size distribution.
U.S. Pat. No. 4,166,026 teaches a two-step process wherein a heavy hydrocarbon oil containing large amounts of asphaltenes and heavy metals is hydrodemetallized and selectively cracked in the first step over a catalyst which contains one or more catalytic metals supported on a carrier composed mainly of magnesium silicate. The effluent from the first step, with or without separation of hydrogen-rich gas, is contacted with hydrogen in the presence of a catalyst containing one or more catalytic metals supported on a carrier, preferably alumina or silica-alumina having a particular pore volume and pore size distribution. This two-step method is claimed to be more efficient than a conventional process wherein a residual oil is directly hydrodesulfurized in a one-step treatment.
U.S. Pat. No. 4,392,945 discloses a two-stage hydrorefining process for treating heavy oils containing certain types of organic sulfur compounds by utilizing a specific sequence of catalysts with interstage removal of H.sub.2 S and NH.sub.3. A nickel-containing conventional hydrorefining catalyst is present in the first stage. A cobalt-containing conventional hydrorefining catalyst is present in the second stage. The first stage is preferably operated under conditions to effect at least 50%w desulfurization, while the second stage is preferably operated under conditions to achieve at least about 90%w desulfurization, relative to the initial oil feed sulfur of the first stage. This process is primarily applicable to distillate gas oil feeds boiling below about 650.degree. F. which contain little or no heavy metals.
All of the above referenced patents relate to two-stage hydrotreating processes for various hydrocarbon oils utilizing certain advantageous catalysts and/or supports. Some of these patents require removal of H.sub.2 S and NH.sub.3 and others do not. However, none have described a process whereby oils with final boiling points from about 650.degree. F. to 1000.degree. F. and/or oil with a low asphaltene content and with components boiling above about 1000.degree. F. can be hydrotreated with significantly improved catalyst life relative to a single catalyst system. Applicants have found that by using a specific stacked-bed catalyst arrangement containing two different catalytically active compositions, oils with high boiling components (about 650.degree. F.-1000.degree. F.) and/or oil with a low asphaltene content and with very high boiling components (greater than about 1000.degree. F.) can be treated in a single stage hydrotreating process with improved catalyst-system life and/or increased hydrotreating conversions for a given feed stock. This process allows easy conversion of existing catalytic hydrotreating reactors to a stacked bed of specified catalysts. The process operates well at hydrogen pressures below about 1100 psig, so that no additional high pressure reactors need be constructed. The particular stacked bed combination of catalysts of the invention results in longer runs between replacements or regenerations for a given oil than would be experienced with either catalyst used alone. Alternatively, poorer quality oils can be processed at equivalent conversions or higher conversions for a given oil can be maintained with the same time between replacement or regeneration with the use of this invention. The invention is most useful for situations where rapid catalyst deactivation is occurring.